Previous work showed that Rathke's pouch (R.P.) tissue from 12 day rat embryos tended to produce neoplasms when transplanted to the hypothalamus of adult hypophysectomized female rats. These growths resembled pharyngiomas, with a large component of adenohypophyseal tissue. Trypsinization of our current pouch tissue transplants has been carried out in an attempt to free the ectodermal element from the mesenchyme. We hope to learn whether pure 12 day R.P. tissue responds to the hypothalamic factors in the same manner as the mixed transplant. Other experiments examine the fate of R.P. tissue from a 12 day embryo, when it is cultured in a millipore chamber of an adult rat's abdomen for 10 days prior to implantation in the hypothalamus of an adult hypophysectomized female rat. Some of the tissue is cultured in the abdomen of hypophysectomized rats, some in intact rats, thus, allowing us to observe tumor propensities in the differently treated brain implants.